In recent years, the number of functions of wireless receivers including radio tuners increases. In the product area of consumer-oriented audio devices and car audio devices, except for the radio tuners, the number of functions including a function for reproducing CDs and DVDs, a connection function to media, such as a USB memory and an SD memory card, and a vehicle-mounted navigation function increases more than ever before, and the lineup of products also increases. These increased functions are often implemented by semiconductor digital circuits that have been highly integrated.
In such a circumstance, there is a problem that harmonic components of various digital signals cause EMI noise and influences a radio frequency range. The EMI noise is sometimes propagated through GND lines or through the space with capacitive coupling and electromagnetic coupling to antenna input lines.
Against the EMI noise that interferes with reception of wireless signals, noise suppression parts such as capacitors and ferrite beads, and methods for avoiding noise by taking measures for circuits that are noise sources have been conventionally studied.
The representative example is described in Patent Literature (PTL) 1 that discloses a technique for spectrum spreading a clock signal of a digital circuit and reducing a peak value of a noise spectrum. Furthermore, PTL 2 describes a technique for shifting a clock frequency according to a reception frequency. These techniques are effective against noise.
On the other hand, digital processing on radio tuners is advancing, and tuner units are being integrated into system large scale integrations (LSIs) using complementary metal oxide semiconductor (CMOS) circuits.
Conventionally, a structure in which an iron plate encloses a small-scale pack module of an analog circuit is mainly used for blocking out external noise.
However, the integration of tuner units into system LSIs allows high-performance digital LSIs and wireless receiving units to be densely mounted on the same board. Thus, the problem of noise is further inextricable in reception of wireless signals.
Along with high speed and high integration, many digital processing circuits including LSIs that implement high functions require memories that have large capacity. Thus, many systems use Dynamic Random Access Memories (DRAMs). The systems are equipped with DRAMs that are near LSIs or included in the LSIs.
The DRAMs store information by accumulating electric charges in capacitors within memory cells. The DRAMs require a refresh operation for reading and rewriting the stored information at predetermined time intervals to hold the information stored in the memory cells.
General DRAMs have a mechanism for holding data by performing refresh access predetermined times in a predetermined period. Specifically, the refresh operation is performed by selecting word lines while the word lines are scanned in the DRAMs.
PTL 3 discloses a technique on a refresh operation to effectively use memories that require refresh. Specifically, a refresh request is performed at predetermined time intervals, and when an access request and a refresh request from a device that uses a memory overlap one another, in principle, processing for the access request is prioritized. In the process, when the number of unprocessed refresh requests reaches a predetermined value, the refresh operation is prioritized.